1. Field of the Invention
The present invention relates to a light emitting element module and a printer head using the same.
2. Description of the Related Art
There has been known one type of light emitting element module in which a plurality of rectangular-parallelpiped chips each including plural LEDs (Light Emitting Diodes) (i.e., "LED array") are arranged on a board (this type of light emitting element module is hereinafter referred to as "LED module").
When an LED is used as a light emitting element, the LED module is used as a light source for an LED print head for use in an electrophotographic printer, for example. On the other hand, when an LED is used as a light emitting element having a photosensitive (photodetecting) function as well as a light emitting function, the LED module is also usable as a reading/writing head for reading and writing characters, images, etc.
FIGS. 1(A) and 1(B) are plan views schematically showing a part of this type of LED module. Particularly, FIG. 1(B) is an enlarged view showing a part of the LED module which is surrounded by a broken line in FIG. 1(A). In FIGS. 1(A) and 1(B), reference numeral 1 represents a board, reference numeral 3 represents a chip and reference numeral 5 represents a light emitting element. When the distance between chips (inter-chip distance) is represented by x and the distance (chip edge margin) between the chip edge and the leftmost (or rightmost) light emitting element (dot) is represented by y in the LED module, the values of x and y are determined in consideration of the following conditions (1) to (3).
(1) The inter-chip distance x is set to such a value as satisfies the following conditions: A) the mutual interference between neighboring chips due to the different in thermal expansion coefficient between the chip and the board can be prevented; and B) even when a chip is mounted on the board or a defective chip is replaced with a new one, the arrangement of adjacent chips can prevented from being disturbed or the adjacent chips can be prevented from being damaged.
(2) The chip edge margin y is set to such a value as satisfies the following conditions: A) there is no probability that the device characteristics can be prevented from being deteriorated due to leakage of light to the side surface at the chip edge; and B) it is easy to cut out (dice) plural chips formed in a wafer individually.
(3) In order to keep the dot pitch constant over the entire LED module, the pitch (dot pitch) z between light emitting elements at the chip edges of adjacent chips and the dot pitch z between light emitting elements on each chip are set to be equal to or approximate to each other.
The values of x and y are determined on the basis of the tradeoffs of the conditions (1) to (3), and the chips are successively mounted on the board by die-bonding or the like to form an LED module.
However, the above-described conventional light emitting element module has such a problem that it is difficult to keep a sufficient inter-chip distance under the condition (3) for a chip having light emitting elements which are arranged in high density.
For example, the following table shows the possible inter-chip distance x for the chip edge margins y of 4 .mu.m and 8 .mu.m when the resolution of the LED module is set to 400 DPI, 600 DPI, 1200 DPI and 2400 DPI.
TABLE 1 ______________________________________ MAXIMUM INTER-CHIP DISTANCEs chip edge margin = 4 .mu.m chip edge margin 8 .mu.m ______________________________________ 400 DPI 55.5 .mu.m 47.5 .mu.m 600 DPI 34.3 .mu.m 26.3 .mu.m 1200 DPI 13.0 .mu.m 5.0 .mu.m 2400 DPI 2.5 .mu.m unmeasurable ______________________________________
It is readily understood from the above table 1 that as the resolution of the LED module is increased, the possible inter-chip distance x is reduced. As the inter-chip distance is reduced, when a chip is arranged or a defective chip is removed, the chip comes into contact with other chips adjacent thereto. Consequently, the probability that the arrangement of the chips is disturbed or the chips are damaged is increased. Therefore, it is more and more difficult to perform a chip arrangement process and a defective chip replacement process. The table 1 shows two cases where the chip edge margin is set to 4 .mu.m and 8 .mu.m. However, upon considering a bonding error, etc., it has been recognized that it is substantially difficult to perform an accurate mounting process for chips having a density (resolution) of 1200 DPI or more even when another chip edge margin value which satisfies the condition (2) is adopted.